This invention relates to a shadow mask sheet made of an Fe-Ni invar alloy, and more particularly to a shadow mask sheet having excellent etchability, a final rolled sheet for a shadow mask sheet, a method of producing such a shadow mask sheet and a cathode ray tube using a shadow mask produced from such a sheet.
Recently, an Fe-Ni invar alloy has been finding wider application in place of aluminum-killed, low-carbon iron materials (AK materials) in cathode ray tubes of television sets and high-resolution displays. However, it is known that the Fe-Ni invar alloy has poorer etchability than the AK materials. Particularly, in recent ultrahigh-precision shadow masks, even slight etching unevenness such as roughness of etched surface and differences of pore shapes would lead to appearance deficiency of shadow masks. Further, a cathode ray tube provided with a shadow mask having such deficiency suffers from the scattering of electron beams, resulting in poor display image.
With respect to shadow mask sheets requiring high-precision etching, proposals have been made in Japanese Patent Publication No. 2-51973 and Japanese Patent Laid-Open No. 61-190023 to reduce impurity elements such as C, O, N, etc. which serve to prevent the etching of the shadow mask sheet, thereby reducing the unevenness of the shadow mask.
As shown in Japanese Patent Publication No. 2-51973 and Japanese Patent Laid-Open No. 61-190023, it is well known that the reduction of the amounts of impurity elements such as C, O, N, etc. which serve to prevent the etching of the shadow mask sheet is the least requirement for improving the etching speed or etching quality of high-resolution shadow mask sheets which should be worked extremely precisely. However, the improvement of evenness of an etched surface and the decrease of differences of pore shapes cannot be obtained only with the reduction of the amounts of impurity elements.
Japanese Patent Laid-Open No. 61-39343 and Japanese Patent Publication No. 2-9655 proposed to improve the alloy structure of a shadow mask sheet for achieving fine, uniform etched pores by controlling the size and orientation of crystal grains. However, in these methods, complicated control in cold rolling and annealing processes is necessary. Also, there are still problems in the roughness and pore shapes in the etched surface, failing to provide highly precisely etched shadow mask sheets. Now that the qualities of cathode ray tubes have been becoming increasingly higher, shadow mask sheets excellent in etchability are strongly demanded.